Fixed issue with sweep conflicting with cashshuffle
Fixed issue with sweep conflicting with cashshuffle

Reported in telegram whereby a user was sweeping a private key but
cashshuffle refused to allow the tx if in "Spend Shuffled" mode.

This has now been fixed to take into account the "tx_external_keypairs"
dict that's an attribute of main_window.

Thanks "CreativeName" for reporting!

fyookball/electrum,fyookball/electrum,fyookball/electrum